Swimming rats have significantly slower heart rates than sedentary controls. When atrial pacing is used to abolish this difference, myocardial contractility is not different in the 2 groups. However, when the aorta is abruptly occluded, exercised hearts develop significantly higher left ventricular systolic pressure and dP/dt and lower end-diastolic pressure than control hearts. Therefore, under stressful stimuli, hearts from conditioned rats perform significantly better than hearts of non-exercising animals. Newer techniques have been used to better quantitate the collateral flow to regions of the canine heart following proximal coronary occlusion. Although the perfusion area of a vessel distal to an obstruction can be defined by injection of a dye such as Evans blue, the blood reaching this area is not solely arriving by collaterals. There is a contribution from normal vessels in adjoining vascular territories. By being able to segregate the circulation to one coronary artery and perfuse the distal portion from a reservoir at normal pressure and flow, it has become possible to measure the extent of this non-collateral contribution, and hence more accurately determine true collteral flow to myocardial regions.